(a) Field of the Invention
The present invention relates to a light-emitting semiconductor device, and more particularly to a light-emitting semiconductor device having a plurality of compound semiconductor layers or compound mixed crystal semiconductor layers formed on a semiconductor substrate for radiating incoherent light.
(b) Description of the Prior Art
Light-emitting semiconductor devices are usually formed with compound semiconductors or compound mixed crystal semiconductors, which impose various limitations on the manufacturing processes, and, hence on the structure, of the light-emitting semiconductor device to be produced.
In light-emitting diodes, substrates are usually formed with compound semiconductors and used to serve as an ohmic region for leading-out an electrode. A diode structure formed with a p type semiconductor region and an n type semiconductor region is formed on a substrate.
Description will hereunder be made of some typical examples of conventional light-emitting diodes.
A GaAsP light-emitting diode is formed with an n.sup.+ type GaAs substrate, an n type GaAsP epitaxial layer formed on the n.sup.+ type GaAs substrate, and a p type region which is selectively diffused in the n type GaAsP layer. The p type diffusion region has a typical impurity concentration of about 1.times.10.sup.18 cm.sup.-3 and is formed to be thin, being below several micrometers, to avoid excessive absorption of emitting and progating light in this p type region. The output light is extracted from that side of this p type region located opposite to the substrate.
A GaP light-emitting diode is formed with an n.sup.+ type GaP substrate, an n type epitaxial GaP layer and a p type epitaxial GaP layer, these latter two epitaxial layers being successively grown on said n.sup.+ type GaP substrate. The p type layer has a typical impurity concentration of about 1.times.10.sup.18 cm.sup.-3. The output light is extracted at the side of this p type layer.
A GaAs light-emitting diode has a similar structure as that of the above-mentioned GaP light-emitting diode. The p type surface layer has a typical impurity concentration of about 2.times.10.sup.18 cm.sup.-3. The output light is extracted from either at the side of the p type surface layer or at the side of the n.sup.+ type substrate (in this latter case, the substrate is partially etched away at sites where the output light is to be extracted).
A GaAlAs light-emitting diode is formed with a p.sup.+ type GaAs substrate, a p type Ga.sub.1-x Al.sub.x As epitaxial layer formed on the GaAs substrate, and an n type Ga.sub.1-y Al.sub.y As epitaxial layer formed on the p type epitaxial layer. The compositions (mixing ratios) x and y are selected to satisfy the condition x&lt;y so as to effectively extract the output light at the side of the n type epitaxial layer. The n type epitaxial layer has a typical impurity concentration of about 1.times.10.sup.17 cm.sup.-3.
In these prior art examples, the substantial part, or the diode structure, of the light-emitting diode is formed with a single n type region and a single p type region which constitute a pn junction therebetween, and it is formed on a substrate. Such structure is relatively easy to manufacture because of its simplicity. The light-emitting efficiency of such LED, however, is inherently limited by this simple structure of the light-emitting diode.
There are also some proposals for improving the light-emitting efficiency of such LEDs by the adoption of an inner structure in the region located between the pn junction and the substrate.
When the radiative semiconductor region has a composition different from that of the substrate, a buffer layer for absorbing lattice mismatch and/or lattice strain may be formed on the substrate.
Iwamoto et al proposed, in Japanese Laying-open Preliminary Patent Publication No. Sho 54-53978 (laid-open on Apr. 27, 1979), a GaP:N green-light emitting diode in which an n type GaP layer and a p type GaP layer are formed successively on an n type GaP substrate. The donor concentration of the n type layer is changed stepwise to form an n.sup.+ type layer on the substrate, and to form an n.sup.- type layer on this n.sup.+ type layer. Nitrogen is heavily doped solely in the n.sup.- type region adjacent the p type surface layer. Iwamoto et al refer to IEEE Trans. on Electron Devices, ED-24, No. 7, pp 951-955 (Beppu et al) in stating that the doped nitrogen atom concentration and the donor concentration are mutually related. They teach, on the basis of this article, that the light-emitting efficiency can be improved by decreasing the donor concentration and by increasing the nitrogen atom concentration at sites located near the pn junction. The intermediate n.sup.+ type layer between the radiative n.sup.- type layer and the substrate is recommended as a kind of buffer layer. This technique, however, only applies to the nitrogen-doped GaP light-emitting diode.
Dumke et al, in U.S. Pat. No. 3,267,294, proposes a liquid nitrogen coded switching device (i.e. operated at the liquid nitrogen temperature), using a deep level impurity provided by Mn or other transition metals. However, the deep level of a transition metal is not suitable for use as the light-emission center of the diode, because it generally has predominant function of serving as a non-radiation center at room temperature. Moreover, even if an emission of light occurs, the light tends to have a prolonged wavelength. Thus, as compared with the emission of light across those bands with which a shallow impurity is associated, the visibility is particularly poor.
Also, Umeda, in U.S. Pat. No. 3,634,872 proposes the provision of a potential to repel carriers injected across the PN junction from the junction by increasing the impurity distribution on that side of the PN junction which is associated with the emission of light. If there are a multiplicity of defects at the interface of the PN junction, the Umeda method is effective. However, in superior PN junctions produced by superior growth methods such as the liquid phase growth method using temperature difference technique, defects at the PN interface are reduced, and the Umeda technique is effective.
Kressel, in U.S. Pat. No. 3,537,029, proposes the employment of PP-junctions. It should be noted, however, the forbidden band width is different between the P type layer and the P-type layer. As noted above, a hetero-junction having different forbidden band widths gives rise to strain or dislocation due to the difference in the lattice constants at the boundary of the hetero-junction, and this constitutes the cause of non-radiation centers.
Rupprecht et al, in U.S. Pat. No. 3,600,240, proposes the employment of amphoteric silicon as an impurity for a PN junction. However, where a PN junction is formed with an amphoteric impurity, it is very difficult to control the carrier concentration. Accordingly, it is difficult to set the value of the impurity concentration at a required value. In general, owing to the intensive compensation, there is a tendency to become a pin junction.
Herzog, in U.S. Pat. No. 3,617,820, proposes to form a P.sup.+ PN junction by diffusing Zn from the source ZnAs.sub.2 into an N type GaAs, to produce dual diffusion profiles. However, where two layers are formed through one diffusion, it is difficult to control the carrier concentrations of the two layers independently of each other.